Cells undergoing programmed cell death (i.e. apoptosis) are recognized by phagocytes such as macrophages and immature dendritic cells (Albert, M. L., et al., “Immature dendritic cells phagocytose apoptotic cells via alpha v beta 5 and CD36, and cross-present antigens to cytotoxic T lymphocytes,” J. Exp. Med. 188(7):1359 (1998)). This recognition leads to the uptake and degradation of the dying cells.
Some of the molecular details of this recognition are now known. For example, recognition of and adherence to apoptotic cells by phagocytes occurs by several mechanisms including a CD36-dependent mechanism (Albert, M. L., et al., (1998)). CD36 is a cell surface glycoprotein that is expressed on dendritic cells, monocytes, and macrophages (Platt, N., et al., Proc. Natl. Acad. Sci., 93: 12456 (1996)). Furthermore, CD36 is a receptor for thrombospondin. (Asch et al. “Isolation of the thrombospondin membrane receptor,” J. Clin. Invest., 79:1054 (1987)).
Thrombospondins are a family of extracellular matrix adhesive proteins. Thrombospondin 1 (TSP 1) inhibits angiogenesis and modulates endothelial cell motility, adhesion, and cell growth. Thrombospondin 1 has multiple functional domains, including a type I repeat which has homology with properdin (Arch, A. S., et al., Biochem. Biophys. Res. Commun., 182(3):1208 (1992); Crombie, R., et al., J. Exp. Med., 187(1): 25 (1998); Magnetto, S., et al., Cell Biochem. Funct, 16(3): 211 (1998); Carron, J. A., et al., Biochem. Biophys. Res. Commun. 270(3): 1124 (2000); and Li, W., et al., J. Biol. Chem., 268:16179 (1993)). Within the type I repeat are two CSVTCG (SEQ ID NO.: 11) sequences that serve as binding sites for CD36. (Pearce, S. F. A., et al., “Recombinant GST/CD36 fusion proteins define a thrombospondin binding domain: evidence for a single calcium-dependent binding site on CD36,” J. Biol. Chem. 270: 2981 (1995); and Asch, A. S., et al., “Thrombospondin sequence motif (CSVTCG; SEQ ID NO.: 11) is responsible for CD36 binding,” Biochem. Biophys. Res. Commun. 182: 1208 (1992)).
Engulfment of apoptotic bodies by phagocytes, including dendritic cells, is mediated by CD36 and results in cross-presentation of antigens to cytotoxic T-lymphocytes. (Albert M. L., et al. (1998)). Furthermore, human monocyte-derived macrophages phagocytose apoptotic neutrophils and eosinophils through a thrombospondin/CD36 dependent mechanism. (Stern et al., “Human monocyte-derived macrophage phagocytosis of eosinophils undergoing apoptosis. Mediation by alpha v beta 3/CD36/thrombospondin recognition mechanisms and lack phlogistic response,” Am. J. Pathol. 149(3):911 (1996)). Therefore, CD36 and thrombospondin play an important role in the recognition and phagocytosis of apoptotic cells.
Classical vaccine technology has included the development of both live and inactivated vaccines. Live vaccines are typically attenuated non-pathogenic versions of an infectious agent that are capable of priming an immune response directed against a pathogenic version of the infectious agent. In recent years there have been advances in the development of live recombinant vaccines (e.g., recombinant poxviruses) in which foreign antigens of interest are expressed from a viral vector. Although there are numerous examples of live vaccines that are very effective (e.g., vaccinia in the eradication of smallpox), there are inherent risks associated with live vectors. For example, it is possible to contaminate live vaccines with harmful adventitious agents, since live vaccines cannot be subjected to harsh inactivation or purification procedures. Additionally, there is a possibility when using live vaccine vectors, of “runaway vaccination” causing systemic viremia in immunocomprised recipients. Finally, live viral vectors elicit strong inflammatory and immunogenic responses against vector components that limit the utility of repeat administration.
Inactivated vaccines are comprised of killed whole pathogens, or soluble proteins or protein subunits. While generally considered safe, the efficacy of inactivated vaccines at eliciting broad, long-lasting responses is of concern for some vaccine preparations. In fact, most inactivated vaccines fail to produce a significant CD8+ cellular response necessary for cytolytic immune activity. Recombinant proteins are promising inactive vaccine or immunogenic composition candidates because they can be produced at high yield and purity. However, recombinant proteins can be poorly immunogenic. Therefore, there is a need for methods and compositions that enhance the immune response to inactivated vaccines, especially vaccines containing recombinant proteins.
Adjuvants have been used for many years to enhance the immune response to antigens present in vaccines, including subunit or component vaccines comprised of recombinant proteins. Currently, alum is the most commonly used adjuvant for human administration. However, although its efficacy has been established, it is ineffective for certain vaccinations (e.g. influenza vaccination) and inconsistently elicits an immune response with certain immunogens. Therefore, there remains a need for safe, effective, and easily manufactured compositions and methods for enhancing an immune response.
Recently, nucleic acid vectored vaccines (NAVAC) have been developed. These vaccines involve direct inoculation of an organism with nucleic acid vectors containing inserts encoding antigens (i.e. genetic vaccination). It appears that both the method (e.g. intramuscular injection, epidermal Gene Gun-mediated administration) and route of inoculation (e.g. intramuscular, epidermal, mucosal) are important in determining the efficacy of the immune response for NAVAC (Robinson et al., Vaccine 11:957 (1993); Ulmer et al., Vaccine 15:792 (1997); and Shiver et al., Vaccine, 15:884 (1997)). This has led to inconsistent stimulation of an immune response using NAVAC. Therefore, there remains a need for effective methods for using NAVAC to consistently elicit a strong immune response.
Despite our increasing understanding of the complex cellular and molecular interactions involved in certain aspects of an immune response, there remains a need for improved methods and compositions, such as vaccines, for enhancing an immune response. More specifically, there remains a need to develop methods and reagents that utilize our growing understanding of antigen presenting cell-recognition molecules, such as CD36, and pathways that depend on these molecules, such as CD36/thrombospondin-dependent pathways.